Photoelectron Spectroscopy
• Lecture 10: Gas phase vs solid/solution
phase measurements
– Gas phase vs solid phase UPS
– Gas phase ionization vs solution phase
oxidation
Why gas phase spectroscopy?
• Allows us to study isolated molecules.
• This is the way we usually visualize electronic structure
of molecules.
• We of course know that solution, environment is a factor,
but how big a factor?
• Need to consider relative strength of environmental
factors compared to intramolecular forces.
Solvation Effects
Gas phase
+
Condensed phase
M+
IE (M)
+
- - +
+
ΔEsolvation (M+)
+
+
M+solv +
+
IE (Msolv)
M
ΔEsolvation (M)
Msolv
Measuring the donor strength of phosphines
Solution-phase IR (νCO) of Ni(CO)3L1 …
PMe3
PPh3
cm-1
2064.1
2068.9 cm-1
O
O
C
C
R3
C
O
… and the pKa of the conjugate acid (PR3H+)
in aqueous solution.2
PR3-H+
PR3 + H+
PMe3
PPh3
7.85
3.05
PMe3 is a better donor than PPh3
(1) Tolman, T.A. J. Am. Chem. Soc. 1970, 92, 2953.
(2) Shaw, B.L. J. Chem. Soc., Chem. Comm. 1979, 104.
Measuring the donor strength of phosphines
Gas-phase proton affinities1….
PR3 + H+
PMe3
PPh3
PR3H+
223.5 kcal/mol
226.7 kcal/mol
…and gas-phase PES studies.2
PMe3
PPh3
- 0.74 eV
8.57 eV
7.83 eV
PPh3 is a better donor than PMe3
(1) Ikuta, S., et al. J. Am. Chem. Soc. 1982, 104, 5899.
(2) Bancroft, G.M., et al. Inorg. Chem. 1986, 25, 3675.
Trend in donor strength is
reversed with different phases.
Preparation of thin films
Au d
Bare Au
Condensed phase samples were
prepared by vapor deposition…
Au Fermi
4A
Mo(CO)4dppe
16 A
50 A
Au substrate
100 A
16
12
8
Ionization Energy (eV)
4
…with an optimum film
thickness of 50-150 A.
Photoionization of a Solid:
What has to happen for a photoelectron to escape?
3) Penetration through
the surface
2) Transport
to the surface
1) Photoexitation
of the electron
EVB
Evac

EF
EVB
Valence Band
bulk
vacuum
surface
0
kinetic energy
Escape Depths: The Universal Curve
Comparison of dmpe and dppe substitution in
the gas and condensed phase
Me
Me
Me P
Gas phase
OC
1.1 eV
Condensed
phase
P
Me
CO
Mo
OC
CO
dmpe
Ph
Ph
Ph P
OC
0.2 eV
Mo
OC CO
10
9
8
7
6
Ionization Energy (eV)
5
P
Ph
CO
dppe
Electrochemistry
• Oxidation: also removal of an electron:
– M  M+ + e-
• Solvation effects.
• Timescale is much slower than photoionization,
allows geometry changes to occur.
• Measure an adiabatic value, not vertical.
• Reference is reference electrode, or internal
reference such as ferrocene oxidation.
• Different definition of second ionization/oxidation.
Relationship Between Ionization and Oxidation
Fc
Fc+ + e-
E(V)
Fc
Fc+ + e-
G
Fe
G = -nFE
Correlation of Ionization and Oxidation
Energies for Substituted Anthracenes
2.2
2
1.8
1.6
1.4
E1/2=0.86(IE)-4.91
1.2
R2=0.97
1
7
7.2
7.4
7.6
7.8
8
Ionization Energy (eV)
Masnovi et al., Can. J. Chem. 1984, 62, 2552.
8.2
8.4
E1/2 vs Ag/AgNO3 (CH3CN)
Correlation of Ionization and Oxidation
Energies for 1,2-Dithiins
0.8
0.7
0.6
0.5
R2 = 0.02
y = 0.0373x + 0.3259
0.4
7.4
7.6
7.8
8
Ionization Energy (eV)
E-C Mechanism:
R. S. Glass et al. JACS 2000, 122, 5052-5064
8.2
Summary
• Important to think about environmental
effects.
• Gas phase and solid/solution phase
measurements do not always show the
same trends.